Pulse generator



DeC- 3, 1946- J. c. SCHELLENG PULSE GENERATOR Filed April 2l, 1944 MAGIVE TRON 2 fl@ F VOLTAGE TIME r\ I l I l \\/voL TAGE l@ (/N Fla. l)

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/Ni/ENTOR J C. SCHELLENG ATTORNEY Patented Dec. 3, 1946 PULSE GENERATOR.lohn C. Schelleng, Interlaken, N. J., assigner to Bell TelephoneLaboratories, Incorporated, New York, N. Y., a corporation of New YorkApplication April 21, 1944, Serial No. 532,118

(Cl. Z50-27) 13 Claims.

This invention relates to electrical pulse generators and particularlyto such generators in which a capacitor is charged from a surge oi highvoltage produced by interrupting the flow of direct current through aninductance coil and then the high voltage of the capacitor is utilizedby discharging the capacitor to the eXtent desired into a load circuitto produce a pulse of high voltage direct current power either forenergizing a high frequency radio transmitter to transmit a short, highpower pulse of radio frequency energy or for some other purpose. Since acommon use for such a device is to produce series of pulses of radiofrequency energy it will be described in that connection. It will beobvious that the invention may also be useful in other applications.

The principal object of the invention is to conserve the energyremaining in the capacitor at the end of a working pulse, that is, afterthe charge of the condenser has been utilized to the extent desired inproducing a direct current power pulse. This is accomplished byproviding a unidirectionally conducting current path between thecapacitor and the direct current source whereby oscillatory discharge ofthe capacitor is damped and the remaining stored energy is returned tothe direct current source.

Another object is to provide a circuit of the impulse type producing ahigh voltage from a relatively low voltage energy source and in whichthe energy pulse may be desirably shaped.

As mentioned above an important use of pulse generators is to energizeradio transmitters in a manner to transmit pulses of radio frequencyenergy. Various types of electron tubes may be employed in suchtransmitters and energized by the pulse generator. One type commonlyused is the magnetron. It is well known that in electron dischargedevices such as the magnetron the electron discharge current falls offvery rapidly as the voltage is reduced. This is illustrated in Fig. 2showing the general nature of a magnetron characteristic. The usefulpart of the characteristic is the upper part roughly indicated as from Ato B. It can be seen that when the applied voltage is a pulse producedfrom the discharge of a capacitor as in the type of pulse generatorunder discussion it may not be desirable to allow the capacitor toapproach complete discharging during a pulse because at the lowervoltages so little power is delivered. Also, when a very short pulse isrequired the capacitor may not have time to discharge appreciably duringthe period of the pulse, In either of these cases there may beconsiderable energy remaining in the capacitor at the end of a workingpulse which is ordinarily dissipated in an oscillatory discharge orotherwise. This is wasteful of energy and may be objectionable in otherrespects.

By this invention the residual energy in the capacitor at the end of aworking pulse is returned to" the low voltage source and therefore notwasted or objectionably dissipated. This is accomplished by providingcurrent paths which permit the energy in the capacitor to return to theinductance coil and then in another step to the original energy source.The invention is explained in more detail by the following descriptionand the accompanying drawing in which:

Fig. 1 is the circuit arrangement, and

Figs. 2, 3 and 4 are explanatory diagrams.

Fig. 1 illustrating the circuit of the invention shows three electrontubes designated I, 2 and 3. Tube I is a triode in which the currentpath between the cathode and anode may be made either substantially anopen circuit or a relatively low impedance by varying the electric lpotential of the grid to make it respectively more negative or morepositive with respect to the cathode potential and its magnitudesuitably related to the potential applied to the anode. The manner of sousing a triode as an electric switch is well known. Tube 2 is atwo-element diode or valve which provides a unidirectional conductingpath presenting always a substantially open circuit to potentialsapplied in one direction and a relatively low impedance circuit topotentials applied in the other direction. Tube 3 is shown as amagnetron of which the evacuated envelope comprises an electricallyresonant cavity within which high frequency waves may be generated andfrom which high frequency energy may be delivered to a load circuitthrough the shielded lead I2 which forms a coupling loop within thecavity in a well-known manner. A tube of this general type is describedin United States Patent 2,063,342, issued December 8, 1936, to A. L.Samuel. The showing of tube 3 in Fig. 1 is in schematic form as it ismerely to illustrate a typical load for the pulse generator. The outputterminals of the pulse generator proper may be considered to be at theterminals of the inductance coil I which are designated for reference I3and I4. Any other load which it may be desired to pulse may be connectedto the terminals I3 and I4 in place of the tube 3. Obviously anythingconnected between the points I3 and I4 3 may be regarded as load and theinductor I may be considered either as part of the load or as anotherpart of the circuit.

Other circuit elements of Fig. 1 are: direct current energy source il,inductance coil of relatively high inductance so that a high voltage isinduced. in it when the current through it is sharply reduced, capacitorl5 which is charged to a high voltage from the inductance coil 5 anddelivers high voltage pulses to the load circuit such as tube S,inductance coil I which is of relatively low inductance and provides alow fre*- quency current path for charging and discharging the capacitori3 but sustains the high voltage of a short pulse. direct current source8 and re.- sistor 9 for biasing the grid of tube I, and the square wavegenerator l l with blocking. condenser It' for timing and initiating thepulses by varying the potential ci the grid of tube I. Suitable meansfor heating the cathodes of tubes i, 3 and are required but forsimplication oi the diagram are notshown. They may be provided invarious known manners. It may be pointed out, however, that the cathodeheating means should not introduce a large capacitance across the tube 2to absorb energy from the high voltage pulse.

The operation of the circuit is as follows: Refer first to Fig. 3 whichshows the operation of the square wave generator `on the grid voltage oftube I. -This figure is a plot of voltage against time. The zerohorizontal base-line from which voltages are measured is near the top ofthe graph. The horizontal broken line marked cut-ofi voltage indicatesthe negative grid voltage required to bias .the tube I to cut-on" forthe particular anode voltage applied from source d. The solid line curveshows the voltage V1 between the grid and the cathode which results fromthe combination of the voltage from the bias source 3 and the voltagefrom the square wave generator I I. Its shape is that of the generatorvoltage and it may be noted that in this particular showing this curveis generally below the zero voltage base-line, varying above and belowthe line of cut-off voltage and rising above the Zero Voltage line onceeach cycle. Various points of interest along the time scale areindicated at C, D, E, E and C. It may be noted that a complete cycle ofoperation extends from C to C".

Fig. 4 shows the variations with time of the current through coil 5 ofFig. i and of the voltage Vz across the tube 2 of Fig. l. The time scaleand the .points indicated on it .are the same as in Fig. 3 with theaddition of points F and G. At C (the beginning of a cycle) thegenerator iI causes .the grid voltage V1 of tube I to rise above thecut-ofi voltage so that current begins to iiow through the coil 5 andthe tube I from the source li. This current increases energizing thecoil 5 until the time D when the generator voltage reverses and thevoltage V1 impressed on the grid of tube I drops below cut-off so thatthe ow `of current through tube I is stopped. This opening of thecircuit through the tube I in effect connects the capacitor E and coil'I in the place of tube I, in series with the coil 5k and source il, andpermits the transfer of energy from coil t to the capacitor E5.Y lt maybe noted that with the circuits through .tube I andtube 2 open thecapacitor 5 and coil 5 are connected together through .the potentialsource l and coil 'I to form a low frequency resonant circuit. Thesource d and coil 'I are unimportant in'this connection on account oftheir low impedances at the resonant frequency. The

.` inductance` coil 7.

4 .transfer of energy from the coil 5 tothe capacitor 6 which starts atthe Itime D, therefore, is the beginning of an oscillatory interchangeof energy between this coil and capacitor which may persist until .theenergy is utilized or dissipated. Due to the inductance of coil 5 thecurrent continues therethrough and through coil 'l during the periodbetween D and E (Fig. 4) charging the capacitor E and by the time .thecurrent has decreased .to zero at E (or approximately E) a high voltageis developed across the capacitor 6. During .the period from C to Eneither tube 3 nor tube 2 has been in operation, -tube 3 because of lackof anode voltage (it being short-circuited by the coil i for directcurr-ent or relatively low frequencies) and tube 2 because its anode hasbeen negative with respect to .the cathode. At E about the time that V2reaches its maximum the voltage of generator I I again reverses (asshown in Fig. 3) driving the grid voltage V1 positive (above cut-ori) orat least in that direction so that the tube i again becomes conductingwith a relatively low impedance and allows the capacitor 6 to dischargewith a steep wave front through the .tube 3 causing it to operate anddeliver high frequency energy through the output lead I2. Due to thesteep wave front the energy is not at once diverted from the .tube 3 bythe shunting coil i through which the capacitor was charged during thevperiod DE. At E a short time (for instance one microsecond) after E thegenerator voltage again reverses and the grid voltage V1 is driven belowcut-off so that .the capacitor discharge through tube 3 is stopped andthe voltage V2 (Fig. e) returns to a value nearly its peak value at E.During the interval EE a pulse of high frequency energy has beendelivered by the tube 3 as the result of a pulse of high voltage energybeing delivered to it by the pulsing circuit. In this case the tube 3 isthe load circuit of the pulsing device Iand obviously the high voltagepulse could be similarly delivered to any suitable load circuit. Theproblem now is to arrange for recharging the capacitor 6 to its peakvoltage for delivering a subsequent high voltage pulse. It is in thispart of the cycle that the particular advantages of the applicantscircuit and method of operation are realized. The situation (as denotedat the time E) is .that the capacitor is still charged to a relativelyhigh voltage and such that it may discharge back through coil 5, sourcei and coil l. None of the tubes is operative. Tube l is biased belowcut-off, tube 2 is not poled to pass current and .tube 3 is eiectivelyshort-circuited by the low An oscillatory discharge therefore startsbetween the capacitor 6 and the coil 5 through the circuit includingalso .the source Il and the coil 1. After one-quarter cycle of thisnatural oscillation (at F Fig. 4) the capacitor 6 is discharged and amaximum of current is built up through the coil 5 in a directionopposite to that at D, that is, in such a direction as to feed energyback into the source 4. At .this .point for the rst time the voltage. V2across the tube 2 is in the direction to permit current to flow through.tube 2. Since tube 2 has a very low impedance in the conductingdirection it substantiallyshort-circuits the-capacitor 6 and coil I andallows the energy stored in the coil 5 to flow into the source e withoutmaterially recharging the capacitors and when none remains in the coilthe current in the coil and ytube 2 becomes zero and the Vtube 2 becomesnon-conducting. This occurs at the time G on Fig. 4. The tube 2 by thuspreventing a substantial recharge of the capacitor G damps theoscillation between the capacitor and coil and causes energy remainingin the capacitor after the working pulse to be returned to .the source4. Substantial natural oscillations between the coil 5 and capacitor 6are thus restricted to .a period of about one-half cycle (between D andF of Fig. 4). Without further oscillation the voltage V2 would thenassume the steady voltage of the source il as it was prior to the time Cand as indicated by the horizontal broken line between G and C on Fig.4. Actually there will be a small oscillation remaining between thecapacitor 6 and the coil 5 as shown by the broken line wave depictingthe voltage V2 and the solid line wave depicting the -current in coil 5between G and C' in Fig. 4. This however represents a relatively smallamount of energy and even it may be recovered (if it is not dissipatedin circuit resistance) by timing the beginning of the next cycle ofoperation to occur when conditions are as shown at C' where the currentand ythe voltage V2 are both substantially zero.

As previously mentioned the operating cycle described is completely atthe time designated C' on Fig. 4 where the conditions are the same as atC and a similar succeeding cycle may start.

The curves of Figs. 3 and 4 are solely for eX- planatory purposes. Theyare drawn and proportioned in a manner to facilitate the explanationgiven of the circuit operation. The relative proportions therefore havenc other signicance. For instance, the time interval EF. is made largeenough to be distinguishable though actually it may be so short inproportion to the other intervals shown that it would beindistinguishable if drawn to the same scale.

The invention, a circuit arrangement comprising means for generatinghigh voltage electrical pulses wherein high voltage energy remaining ina capacitor at the termination of a pulse is recovered has beendescribed in connection with a magnetron type of high frequencygenerator t0 which the pulses are delivered for the purpose of producingpu'ses of high frequency energy. It is o-bvious that the arrangement maybe used with any type of high frequency generator which may be soenergized and also as a source of high voltage pulses for any otherappropriate purpose.

t is intended therefore that the invention is not limited by theparticular specic disclosure but only by the appended claims.

What is claimed is:

1. The method of producing and utilizing pulses of electrical energywhich comprises the steps of transferring electrical energy from asource to a reactive element of a resonant circuit, transferring part ofsuch transferred energy to a load circuit in a pulse shorter in timethan one-fourth of the natural period of the said resonant circuit andreturning a substantial part of the energy then remaining in thereactive element back to the source.

2. In a start-stop electrical circuit having a cycle of operation, andcomprising a source, a load circuit and a resonant circuit including areactive element, control means responsive to signals producedindependently of the said resonant circuit ror determining the Saidcycle of operation, means comprising said control means for transferringenergy from the source to the reactive element in the rst part of thecycle, means comprising the said control means for delivering a portionof the said energy to the said load circuit in a pulse shorter in timethan one- 6 fourth of the natural period of the said resonant circuitand means for restoring part of said energy to the source in the latterpart of the cycle.

3. A circuit comprising a source of voltage and an inductance, means forapplying said source of voltage to said inductance whereby energy isstored in the inductance, means for coupling said inductance with acapacitance to form a resonant circuit whereby a relatively high voltageis momentarily produced across said capacitance, means for applying thesaid momentarily produced high voltage to a load circuit for a periodless than one-fourth the natural period of the said resonant circuitwhereby a portion of the said energy is transferred to the load circuitand means whereby a substantial part of the remainder of said energy isrestored to said source of voltage.

4. A circuit comprising an electrical source, a reactance element, meansfor applying said electrical source to said reactance element wherebyenergy is stored in the reactance element, a second reactance element,means for coupling the second reaetance element with said iirst namedreactance element to form a resonant circuit and to initiate anelectrical oscillation in the resonant circuit, means for connecting aload circuit to thesaid resonant circuit during the persistence of thesaid electrical oscillation for a period less than onefourth the periodof the oscillation whereby a portion of the said energy is delivered tothe load circuit and means for restoring part of said energy to saidelectrical source.

5. A circuit comprising an electrical source, a reactance element, meansfor applying said electrical source to said reactance element wherebyenergy is stored in the reactance element, a second reactance element,means for coupling the second reactance element with the first namedreactance element to form a resonant circuit and to initiate anelectrical oscillation in the resonant circuit, means for connecting aload circuit to the said resonant circuit during the persistence of thesaid electrical oscillation for a period less than one-fourth the periodof the oscillation whereby a portion of the said energy may be deliveredto the load circuit and means comprising a unidirectionally conductingdevice connected effectively in parallel with one of the reactanceelements for restoring at least part of said energy to said electricalsource.

6. A circuit comprising an electrical source, a reactance element, meansfor applying said electrical source to said reactance element wherebyenergy is stored in said reactance element. a second reactance element,means for coupling the second reactance element with said rst namedreactance element to form a resonant circuit and to initiate anelectrical oscillation in .said resonant circuit, means for connecting aload circuit to the said resonant circuit during the persistence of thesaid electrical oscillation for a period less than one-fourth the periodof the oscillation whereby a portion of the said energy may be deliveredto the load circuit and means for substantially suppressing theelectrical oscillation in the resonant circuit comprising the tworeactance elements before the completion of a cycle and for returning asubstantial part of the oscillation energy to the said electricalsource.

7. A circuit according to claim 6 in which the means for suppressing anelectrical oscillation in the resonant circuit and returning asubstantial part of the oscillation energy to the electrical sourcecomprises a unidirectonally :conducting device effectively shunting oneof the reactance elements.

8. A pulse generator compri-sing a source of direct current, aninductor, a capacitor, and a load circuit capable of passing directcurrent in either direction connected in series with each other in theorder named, a unidirectionally conducting path bridging the said directcurrent source and inductor in series and arranged to be non-conductingto current from the said source, switching means for first establishinga flow of current from the said source through the said inductor, nextinterrupting the iiow of current through the inductor Without openingthe said series connection of the inductor and capacitor, whereby thecapacitor is charged to a high voltage b-y the energy stored in theinductor by the said flow of current from the source, next for closing apath for discharge of the capacitor through the ioad circuit and iinallyfor opening the said discharge path without opening the said seriesconnection of the inductor and capacitor whereby the residual energy ofthe capacitor charge is returned to the inductor and thence through thesaid unidirectionally connecting path to the direct current source.

'9. A pulse generator according to claim 8 in which the load circuitcomprises a pulse energy absorbing member and a member connected inparallel therewith which presents a high impedance to the pulse currentbut' a relatively low impedance to low frequency and direct current.

l0. A pulse generator according to claim 8 in which the load circuitcomprises an inductor connected between the load circuit terminals.

1l. A pulse generator comprising a source of direct current, arelatively high impedance inductor, a capacitor and a relatively lowimpedance inductor arranged in a series circuit in the order named, aunidirectionally conducting current path bridging both the saidrelatively high impedance inductor and the direct current source andarranged to be non-conducting to current from the said source, anelectronic switch connected in parallel with the said unidirectionallyconducting path and arranged when desired to conduct current from thesaid source,v means for controlling the electronic switch in a desiredsequence and a load circuit for the generated pulses connected inparallel with the said relatively low impedance inductor. y

121 A pulse generator of the type in which a capacitor arranged todeliver power to a load circuit in pulses of which the periodicity andduration are determined by a timing Wave is charged to a high voltagebythe interruption of current iiowing from a direct current source throughan inductor and in which a low impedance unidirectional path isconnected to form with the direct current source and said inductor aclosed circuit and to bridge the said capacitor independently of thesaid source and inductor and is so poled as to effectively isolate the'capacitor from the said inductor and source when a voltage is generatedin the inductor of such a polarity as to pass a reverse current throughsaid source.

l?. The method of producing and utilizing pulses of electrical energywhich comprises the steps of transferring electrical energy from adirect current source to a reactiverelement of an electrically resonantcircuit, allowing the said transferred energy to initiate an electricaloscillation in the said resonant circuit whereby there is producedthrough resonance therein a voltage surge, connecting a load circuit tothe said resonant circuit for a limited period during the time oi thesaid voltage surge whereby a pulse of energy is delivered to the loadcircuit from the resonant circuit, the length of the pulse beingshorter` than a period of oscillation of the resonant circuit and tooshort to permit delivery to the load circuit of all the energy in theresonant circuit, and returning a substantial part of the energy thenremaining in the resonant circuit back to the said energy source.

JOHN C. SCHELLENG.

